The present invention relates generally to a cochlear implant auditory prosthesis, and particularly to a multi-channel cochlear implant system which transmits speech information and electrical power across separate transcutaneous paths.
In recent years, a method has been developed for inducing the sensation of hearing in people suffering from sensory deafness. This method involves the direct electrical excitation of the auditory nerve endings distributed along the basilar membrane of the cochlea of the ear. The electrical stimulus is generated by an auditory prosthesis known as the cochlear implant. Cochlear implants generally comprise at least one antenna coil, a receiver, and an electrode. The antenna coil is used to receive radio frequency transmitted signals representing sould from an external inductive coil assembly disposed over the implanted antenna coil. The receiver then detects or demodulates the electrical signal sent by the antenna coil. This processed electrical signal is then transmitted to the electrode which creates an electrical field along the basilar membrane within the cochlea or otherwise distributes the processed signal along this membrane.
In some cochlear implant designs, a single communications channel is established between the external transmitter and the subcutaneous receiver of the cochlear implant. In these systems, one antenna coil receives the radio frequency transmitted sound excitation signal. While in other cochlear implant designs, multiple communication channels are established between the transmitter and the receiver. Multiple channel cochlear systems usually utilize one antenna coil for each of the communication channels.
A further discussion of cochlear implants may be found in the following patents: U.S. Pat. No. 3,751,605, issued on Aug. 7, 1973 to Robin P. Michelson; U.S. Pat. No. 3,752,939 issued on Aug. 14, 1973 to Melvin C. Bartz; U.S. Pat. No. 4,495,917, issued on Jan. 29, 1985 to Charles L. Byers; U.S. Pat. No. 4,400,590, issued Aug. 23, 1983 to Robin P. Michelson. Additional discussions of cochlear implants may also be found in to following publication: "The Functional Replacement of the Ear" by Gerald E. Loeb, Scientific American, Vol. 252, No. 2, dated 2/9/85, pp. 104-111; "Design and Fabrication of an Experimental Cochlear Prosthesis" by Gerald E. Loeb, et al. May 1983, pp. 241-254, Medical and Biological Engineering and Computing. The above identified patents and publications are incorporated herein by reference.
Multiple channel cochlear implant systems are believed to be preferable to single channel systems as a result of the place-pitch theory. In accordance with this theory, the pitch of the sound perceived when the cochlea is stimulated depends upon which portion of the basilar membrane within the cochlea is stimulated. When the base of the cochlea is stimulated, higher pitches are perceived. As the stimulus moves toward the apex of the cochlea, the perceived pitch lowers. It will be appreciated that it is difficult for a single channel system to stimulate discrete protions of the cochlea and thereby stimulate the preception of different pitches. Indeed, some experiments have shown that a minimum of six separate channels of stimulation are necessary to create enough pitch discrimination to simulate intelligible speech.
The number of channels in a cochlear implant system is limited by a number of factors. One limitation is the number of electrodes. Implanted electrodes must be spaced a sufficient distance apart to prevent interaction between adjacent electrode contacts. Because of this, current implants often employ bipolar electrode contacts which provide a more localized pattern of excitation. The size and shape of the cochlea also limits the number of electrodes which may be implanted.
The numbers of channels in a cochlear implant system is also limited by the method employed to transmit electrical signals across the skin. In present multi-channel cochlear implant systems, a separate transmitter/receiver antenna pair is usually employed for transmitting each communication channel. Thus, for example, in the article "The Functional Replacement of the Ear" referred to above, four transmitter/receiver antenna pairs are shown--one for each channel in the cochlear implant design. While this approach is appropriate for cochlear implants with relatively few channels, this approach is difficult to implement for cochlear implants employing six or more channels. This is because the size of the coils places a practical limit on the number of coils that can be attached on both sides of the patient's skin.
Thus, it would be desirable to provide a multi-channel cochlear implant system which requires few or even just one antenna/receiver pair for transmitting all of the channels of information. A significant problem, however, in such a system, would be the increase in the complexity of the signal processing circuitry that would be required by the implanted receiver circuit. This creates difficulties due to the size and power limitations on the implanted device. The power required to drive both the implanted circuitry and the electrodes could necessitate the use of an implanted battery. Such a battery is undesirable because it requires additional space and must be replaced periodically.
Moreover, supplying electrical power to the implant is difficult even in conventional implant designs. A relatively large amount of electrical power is required to drive the implanted electrodes. In conventional cochlear implants often both the audio signal and electrical power are transmitted across a single antenna coil pair. While a wide bandwidth is necessary for effective transmission of the audio signal, this results in large power losses across the cutaneous layer. As a result, not much of the transmitted power is available to drive the electrodes. This sometimes results in excessive drain on the external battery worn by the user. It will be appreciated that a narrow bandwidth signal would be more efficient for transmitting electrical power across the skin. Thus, it would be desirable to provide a multi-channel cochlear implant system which transmits electrical power across a separate antenna using a narrow bandwidth signal.
Besides improving pitch discrimination, having multiple channels allows greater flexibility in modifying the electrical stimulation to improve the perception of speech in the patient. This is because in some patients, portions of the cochlea do not respond to electrical stimulation. Also, malfunctions can occur which limit the functioning of a particular electrode. Thus, for example, in a four channel implant system one or more channels may become non-functional. The conventional approach in such a case would be to combine the audio signals from the non-functioning channel with the signals for the functioning channel. In this way, the full audio spectrum is retained. This results, however, in a reduction in the number of discrete channels of stimulation to three or less. Thus, it would be desirable to provide a cochlear implant with a large number of electrodes so that a greater number of functioning electrodes remain if some of the electrodes become non-functional.
Also, prior multiple channel cochlear implants have generally employed a fixed filtering and processing scheme to divide the full spectrum of audio frequencies into discrete channels to stimulate the individual monopolar or bipolar electrodes. However, since a patients' response to cochlear implants may vary widely, some patients may achieve better results with monopolar instead of bipolar electrodes. This is because the nerve damage in some implant patients may differ in its location and severity in comparison with other implant patients.
Therefore, it would thus be desirable to provide a programmable multi-channel cochlear implant system in which various audio and auditory parameters can be easily optimized after the implant is in place. These parameters include changing from a monopolar to a bipolar electrode configuration, changing the content of each channel with respect to the frequency, changing the bandwidth and phase of the signal, and compensating for non-functioning electrodes. For example, a programmable multi-channel implant would be able to manipulate the incoming signal to the implant in new ways. The patient then could be expected to achieve greater comprehension of speech than was previously possible. An added benefit of such a system would be its utility as a research tool to advance our understanding of the auditory system and thereby further the development of future auditory prosthesis systems.
Accordingly, it is a principal objective of the present invention to provide a multi-channel cochlear implant system which transmits multiple channels of audio information across a single wireless transcutaneous path to produce intelligible perception of speech. It is also a principal objective to provide a multi-channel cochlear implant system which efficiently transmits electrical power to the implant across a separate wireless transcutaneous path so that excessive current is not drawn from the external battery.
It is another objective of the present invention to provide a portable cochlear implant system which has relatively low power requirements, and permits the transmission of speech information over a wide bandwidth and also permits the transmission of electrical power over a narrow bandwidth.
It is also an object of the present invention to provide a programmable multi-channel cochlear implant system in which the audio information contained in each channel can be easily altered after the implant is in place, to tailor the electrical stimulation to the patient's individual needs, and thereby optimize the comprehension of speech.
It is an additional objective of the present invention to provide an implantable receiver circuit which can synchronize itself with the transmitted channels of speech data and also transmit these channels of data to individual implanted cochlear electrodes in a predetermined sequence.
It is also an objective of the present invention to provide a system which can transmit electrical power across the skin to an implanted circuit and to also adjust the frequency of the transmitted signal to optimize the efficiency of the power transmission.